The Vortex category is determined by a/c Max take off weight:
In the UK:
Up to 17000kg is light
17000 - 40000kg is small
40000-136000kg is medium
more than 136000kg is heavy,
The 757 was found to produce more violent vortex wake than other mediums, so we classify it as an 'upper medium' along with Ilyushin 62, VC10, 707 and DC8.
I don't think this has anything to do with airworthiness, it's purely an operational concern.
Gonzo.
[ 06 January 2002: Message edited by: Gonzo ]</p>